Nonaqueous electrolyte secondary batteries typified by lithium ion batteries are widely used as driving power supplies for portable electronic devices such as mobile phones including smartphones, mobile computers, PDAs, and portable music players. Furthermore, the nonaqueous electrolyte secondary batteries have become widely used in driving power supplies for electric vehicles and hybrid electric vehicles and stationary storage battery systems for applications for suppressing output fluctuations in solar power generation, wind power generation, and the like and peak shift applications for grid power for the purpose of storing electricity during nighttime to use electricity during daytime.
However, power consumptions tend to further increase with the improvement of applied devices; hence, a further increase in capacity is strongly required.
Examples of a method for increasing the capacity of the nonaqueous electrolyte secondary batteries include a method for increasing the capacity of an active material, a method for increasing the filling amount of an active material per unit volume, and a method for increasing the charge voltage of a battery. However, in the case of increasing the charge voltage of a battery, the crystal structure of a positive electrode active material is likely to be deteriorated or the positive electrode active material and a nonaqueous electrolyte solution are likely to react with each other.
As a measure against high voltage, Patent Literature 1 reports that a lithium layer is stabilized and the crystal structure at high voltage is stabilized in such a manner that after a positive electrode active material, LiCoO2, is charged at 4.2 V to 4.3 V versus lithium, lithium is partially substituted with magnesium in an electrochemical way.